IdeS (Immunoglobulin G-degrading enzyme of S. pyogenes) is an extracellular cysteine protease produced by the human pathogen S. pyogenes. IdeS was originally isolated from a group A streptococcal strain of serotype M1, but the ides gene has now been identified in all tested group A streptococcal strains. IdeS has an extraordinarily high degree of substrate specificity, with its only identified substrate being IgG. IdeS catalyses a single proteolytic cleavage in the lower hinge region of human IgG. This proteolytic degradation promotes inhibition of opsonophagocytosis and interferes with the killing of group A Streptococcus. IdeS also cleaves some subclasses of IgG in various animals and efficiently converts IgG into Fc and Fab fragments. The ides gene has been cloned and expressed in E. coli as a GST fusion protein.